Porosity has long been recognized as one of the major casting defects affecting mechanical properties, especially fatigue performance, of cast components. Porosity forms due to volume shrinkage from liquid to solid during solidification, and, in particular, due to the evolution of the dissolved gases as a result of the significant decrease in solubility of the gases in the solid as compared to the liquid metal. Hydrogen is the only gas that is appreciably soluble in molten aluminum. (See Q. Han, S. Viswanathan, Metallurgical and Materials Transactions A, 33 (2002) 2067-2072; and D. R. Poirier, K. Yeum, A. L. Maples, Metall Trans A, 18 (1987) 1979-1987.). Thus, reducing or eliminating the dissolved hydrogen in molten Al helps to produce high quality castings.
There are several methods that are currently employed to reduce inclusion and hydrogen content in liquid aluminum. These methods include rotary impeller degassing using nitrogen, argon, or a mixture of the inert gases and chlorine as a purge gas; tablet degassing (such as hexachloroethane (C2Cl6) tablets); vacuum degassing; ultrasonic degassing; and spray degassing. (See A. M. Samuel, F. H. Samuel, J Mater Sci, 27 (1992) 6533-6563; A. C. Kevin., J. H. Michael, Light Metals, (2001) 1017-1020; R. Wu, Z. Qu, B. Sun, D. Shu, Materials Science and Engineering: A, 456 (2007) 386-390; and H. Xu, Q. Han, T. Meek, Materials Science and Engineering: A, 473 (2008) 96-104). Although the existing degassing methods have demonstrated effectiveness to varying degrees in refining Al melts, they can cause environmental problems (for example, due to Cl2 gas release) or involve significant capital investment.